Zolmitriptan (I) chemically known as (4S)-4-[[3-2-(dimethylamino)ethyl]-1H-indol-5-yl]methyl]-2-oxazolidinone, belongs to the triptan family of compounds and is a selective serotonin receptor agonist of the 1B and 1D subtypes. It is used in the acute treatment of migraine attacks with or without aura and also in cluster headaches. Zolmitriptan is available under various brand names such as Zomig, Zomigon, AscoTop, Zomigoro, Flezol etc.

Zolmitriptan was first disclosed in U.S. Pat. No. 5,466,699 and the synthetic route comprises diazotization of (S)-4-(4-aminobenzyl)-1,3-oxazolidin-2-one with sodium nitrite in presence of hydrochloric acid, followed by treatment of the diazonium salt with stannous chloride to give (S)-4-(4-hydrazinobenzyl)-1,3-oxazolidin-2-one. Further reaction of the hydrazine derivative with dimethylamino butyraldehyde diethyl acetal followed by chromatographic purification gave Zolmitriptan (I) as oil, which upon further crystallization from isopropanol gave Zolmitriptan of desired purity. (S)-4-(4-aminobenzyl)-1,3-oxazolidin-2-one, in turn, was obtained from L-4-nitrophenyl alanine by treating it with thionyl chloride and methanol to give (S)-methyl-4-nitrophenyl alanate, which on reduction with sodium borohydride gave the corresponding alcohol. Further reaction of the said alcohol with phosgene in presence of potassium hydroxide followed by hydrogenation in presence of palladium on carbon gave the desired (S)-4-(4-aminobenzyl)-1,3-oxazolidin-2-one.
A significant disadvantage of this method involves the utilization of a relatively large excess of stannous chloride, in the range of about 4.7 molar equivalents during the preparation of the intermediate, (S)-4-(4-hydrazinobenzyl)-1,3-oxazolidin-2-one hydrochloride, which results in formation of large amounts of stannous hydroxide. Also, the hydrazine intermediate thus obtained requires a highly energy intensive procedure of distilling out large volumes of aqueous mass followed by trituration with a low boiling and flammable solvent like ether. The utilization of excess stannous chloride generates high level of associated impurities in the intermediate due to which repeated purification involving column chromatography is required for obtaining the desired purity of Zolmitriptan. Successive purification results in a low yield of about (18% w/w) hence, the method is not preferred for commercial scale.
WO 97/06162 discloses a one-pot process for the conversion of (S)-4-(4-aminobenzyl)-1,3-oxazolidin-2-one to Zolmitriptan (I) wherein the amine derivative is diazotized using sodium nitrite and the resulting diazonium salt is reduced using sodium sulfite to give the corresponding hydrazine derivative, which then requires an abnormally high volume of solvent during reaction with 4,4-diethoxy-N,N-dimethylbutylamine to yield Zolmitriptan.
The process, despite avoiding the use of stannous chloride and isolation of intermediates, resorts to large amounts of sodium sulfite as reducing agent and due to the in-situ mode of reactions, the associated impurities which are generated in significant proportions are carried forward thereby necessitating laborious purification methods to obtain Zolmitriptan with reduced yield.
WO2009/044211 discloses a process for synthesis of Zolmitriptan wherein (S)-4-(4-aminobenzyl)-1,3-oxazolidin-2-one is diazotized using sodium nitrite, followed by reduction of the diazonium salt to the corresponding hydrazine derivative using stannous chloride. Further reaction of the hydrazine derivative with 4,4-diethoxy-N,N-dimethylbutylamine and cyclization gave Zolmitriptan. The specification also discloses a one-pot synthesis method wherein very high dilutions are necessary to minimize the formation of associated impurities generated during the reaction. Due to the in-situ nature of the method, the undesirable side products in the penultimate step is carried forward to yield an impure product, and necessitates a number of purification steps to obtain the final product having desired purity.
Thus, there is a need for a process wherein the associated impurities are minimized and removed during isolation of intermediates without an additional step of purification and wherein such a procedure does not involve energy intensive and time consuming unit processes such as solvent distillation for obtaining either the intermediates or the final product. Therefore, the present inventors in their quest for developing a method which provides intermediates which are substantially free from associated impurities and results in a high-yielding, scalable chemical process for the synthesis of Zolmitriptan (I), have, after rigorous experimentation, developed a method which provides a method for isolation of (S)-4-(4-hydrazinobenzyl)-1,3-oxazolidin-2-one hydrochloride of formula (IIIa) by overcoming the shortcomings of the prior art and helps in providing Zolmitriptan of desired purity.